The heat treatment of metal parts often includes a quenching step in which the metal parts may be cooled from high temperatures to room temperature by immersion in a fluid such as water or oil. Depending upon the material involved, the size of the part and other factors, quench cracking is often encountered. Quench cracking arises as a result of the thermal stresses developed in the part during the cooling process and often occurs at areas of geometrical section change.
Quench cracking is a factor which limits the suitability of heat treatment processes with certain alloys. The modern tendency is to minimize excess material in turbine part preforms, thus even shallow cracking can ruin superalloy components since quench cracks cannot be completely removed by machining without producing an undersized part.
Quench cracks are observed to initiate at sites where prior oxidation has occurred--these sites are often grain boundaries. It might be thought that such cracking might be reduced or eliminated by performing the heat treatment in a vacuum or inert atmosphere. This is not feasible for two reasons, first the reactivity with oxygen of common superalloy constituents (Al, Ti, Hf, etc.) is so high that it is almost impossible to provide an atmosphere with sufficiently low oxygen level to avoid oxidation. The second reason is that inert atmosphere or vacuum furnaces both limit the speed with which the article can be quenched, thus compromising the heat treatment and resultant mechanical properties.
In the general field of heat treatments it is known to use coatings on the surfaces of parts to be heat treated for a variety of reasons. In the carburizing of steel for example, it is known to use localized copper plating to eliminate carburizing of selected surface portions. However, copper is an element known to adversely affect superalloys. In the heat treatment of steel it is also known to use glass based coatings on parts to reduce surface oxidation. Glass base coatings are generally fragile and impractical especially on massive parts.